When tblgen changes, regenerate all .inc files, but do not rebuild any .o

files that USE the .inc file unless the contents of the .inc file changes.

This should fix the problem where reconfiguring causes all targets to be
completely rebuilt (because config.h is usually modified, causing libsystem
to be rebuilt, causing tblgen to be rebuilt, causing .inc files to be
rebuilt, causing .o files to be rebuilt).

This patch also checks in a gross hack where .o files now explicitly depend
on $(BUILT_SOURCES), to avoid problems where the .inc files are not completely
generated before the .o files start to compile.

llvm-svn: 18986